With some materials to be drilled, it is necessary that a drill rotate a drill bit at a slow speed. For example, stainless steel is difficult to machine because of its hardness, toughness and work-hardening properties. Thus, stainless steel requires a slower cutting speed of generally 60 surface feet per minute, which is roughly one-half that of low carbon steel, to minimize friction and heat build-up when using a hole cutter to make a hole. Although variable speed power drills are available, such drills provides insufficient torque at the low speeds to drive some drill bits such as hole cutters and especially larger diameter hole cutters.
In some applications, it is necessary to use two different types of drill bits. For example, in drilling holes, often a pilot drill bit is employed along with a hole cutter. Furthermore, it is sometimes desired that the two drill bits be rotated at different speeds to best accomplished the task that the drill bits are designed to do.